Solid state electrolytes are very attractive for use with lithium-ion batteries since these electrolytes, as compared to liquid-based non-aqueous electrolytes, are non-flammable. The solid state electrolytes have specific ionic conductivities on the order of 10−3 S/cm, which is comparable to liquid-based non-aqueous electrolytes. Furthermore, with a lithium ion transfer number of unity, concentration gradients at high discharge rates can be avoided with solid state electrolytes that otherwise would occur within liquid-based electrolytes under the same operating conditions.
Lithium Phosphorus Oxynitride (“LIPON”), for example, is currently used as a solid-state electrolyte in rechargeable lithium cells operable at ambient temperatures and where high capacity cells are not a requirement. However, LIPON has a low specific ionic conductivity ( i.e., on the order of 10−6 S/cm at room temperature) with high activation energy (Ea) of 53 kJ/mol. Additionally, sputtering of LIPON is required for electrochemical cell integration, which increases the cost and fabrication difficulty for a large area, pin-hole free electrolytes.
Other solid-state electrolytes are currently available with specific lithium ionic conductivities on the order of 10−3 S/cm at room temperature. These include: (1) highly lithium ion conducting glass-ceramic solid-state electrolytes based on Li2S—P2S5 with an Ea of 12 kJ/mol—processing of these materials requires the use of an Argon-filled dry box; (2) thio-LISICON-lithium superionic conductor (Li3.25S—Ge0.25—P0.75S4), Ea of 20-45 kJ/mol as a function of heat treatment temperature; and (3) Ohara glass ceramic, Ea of ˜35 kJ/mol.
Despite these benefits to solid-state electrolytes (including fast lithium ion conduction properties), the integration of solid-state electrolytes into cell components can be impeded by high-impedances at the glass-ceramic solid-state electrolyte/electrode interface. While conventional approaches have used intermediate layers, such as LIPON or polymer based electrolytes, to facilitate connection between the glass-ceramic electrolyte and electrode, these approaches can be costly or introduce additional problems into the battery.